Many Micro Electromechanical Systems (MEMS) include a structure, such as shown in FIGS. 1A and 1B, in which a mass 10 is resiliently mounted above a substrate 12. For example, a comb drive is formed on a proof mass 10 elastically mounted to a substrate 12, having a plurality of teeth 14 extending therefrom. Corresponding teeth 16 are rigidly mounted to the substrate and electrically coupled to signal generating and sensing circuits. The capacitance between the teeth 14 of the proof mass 10 and the rigidly mounted teeth 16 is used to sense movement of the mass 10 and to actuate the mass 10.
In such systems, the proof mass 10 is typically a very small distance from the substrate 12. As a result, it is possible for the proof mass 10 to contact the substrate. Because the forces involved are typically extremely small, adhesion between the proof mass 10 and the substrate 12 due to intermolecular forces, such as van der Waals interactions, become significant. The contacting surfaces of the proof mass and substrate are therefore prone to adhere to one another when brought into contact with one another due to a phenomenon known as “stiction.” Stiction may cause the mass and substrate to fail to separate during the fabrication process. CO2 drying or other drying methods may be used to release stiction during fabrication. However, stiction can still occur during device operation. Sometimes this stiction is permanent and the device then cannot be recovered. In some cases, stiction during device operation is temporary and can be overcome by inertial forces exerted on the mass 10. However, even where stiction is overcome, measurements derived from the device and movements performed by the device will be subject to errors caused by the temporary adhesion of the proof mass to the substrate.
In view of the foregoing, it would be an advancement in the art to provide a MEMS device not subject to errors caused by stiction.